X-rays are widely used in materials analysis systems. For example, x-ray spectrometry is an economical technique for quantitatively analyzing the elemental composition of samples. The irradiation of a sample by high energy electrons, protons, or photons ionizes some of atoms in the sample. These atoms emit characteristic x-rays, whose wavelengths depends on the atomic number of the atoms forming the sample, because x-ray photons typically come from the tightly bound inner-shell electrons in the atoms. The intensity of the emitted x-ray spectra is related to the concentration of the atoms within the sample.
Another example is x-ray fluoroscopy, which is used for chemical analyses of solids and liquids. Typically, a specimen is irradiated by an intense x-ray beam, which causes the elements in the specimen to fluoresce, i.e. to emit their characteristic x-ray line spectra. The lines of the spectra can be diffracted at various angles by a single-crystal plate. The elements may be identified by the wavelengths of their spectral lines, which vary in a known manner with atomic number. The concentrations of the elements in the specimen may be determined from the intensities of the lines. The x-ray fluorescence method has proven to the particularly useful for mixtures of elements of similar chemical properties, which are difficult to separate and analyze by conventional chemical methods.
Typically, the x-rays used for materials analysis are produced in an x-ray tube by accelerating electrons to a high velocity by an electrostatic field, and then suddenly stopping them by collision with a solid target interposed in their path. The x-rays radiate in all directions from a spot on the target where the collisions take place. The x-rays are emitted due to the mutual interaction of the accelerated electrons with the electrons and the positively charged nuclei which constitute the atoms of the target. High-vacuum x-ray tubes typically include a thermionic cathode, and a solid target. Conventionally, the thermionic cathode is resistively heated, for example by heating a filament resistively with a current. Upon reaching of a thermionic temperature, the cathode thermionically emits electrons into the vacuum. An accelerating electric field is established which acts to accelerate electrons generated from the cathode toward the target. A high voltage source, such as a high voltage power supply, may be used to establish the accelerating electric field. In some cases, the accelerating electric field may be established between the cathode and an intermediate gate electrode, such as an anode. In this configuration, a substantially field-free drift region is provided between the anode and the target. In some cases, the anode may also function as a target.
In one form of a conventional x-ray machine, the cathode assembly may consist of a thoriated tungsten coil approximately 2 mm in diameter and 1 to 2 cm in length. When resistively heated with a current of 4 A or higher, the thoriated tungsten coil thermionically emits electrons. In many applications, most of the energy from the electron beam is converted into heat at the anode. To accommodate such heating, high power x-ray sources often utilize liquid cooling and a rapidly rotating anode.
It is desirable that the cathode be heated as efficiently as possible, namely that the thermionic cathode reach as high a temperature as possible using as little power as possible. In conventional x-ray tubes, for example, thermal vaporization of the tube's coiled cathode filament is frequently responsible for tube failure. Also, the anode heated to a high temperature can cause degradation of the radiation output. During relatively long exposures from an x-ray source, e.g. during exposures lasting from about 1 to about 3 seconds, the anode temperature may rise sufficiently to cause it to glow brightly, accompanied by localized surface melting and pitting which degrades the radiation output.
In the field of medicine and radiotherapy, an optically driven (for example, laser driven) therapeutic radiation source has been disclosed in U.S. application Ser. No. 09/884,561, commonly owned by the assignee of the present invention, and hereby incorporated by reference (hereinafter the '561 application). This optically driven therapeutic radiation source uses a reduced-power, increased efficiency electron source, which generates electrons with minimal heat loss. The '561 application discloses the use of laser energy to heat an electron emissive surface of a thermionic emitter, instead of using an electric current to ohmically heat an electron emissive surface of a thermionic emitter. With the optically driven thermionic emitter, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the requisite power requirements.
For materials analysis systems, however, there is a need for miniaturized, increased efficiency x-ray sources. It is an object of this invention to provide a miniaturized, portable x-ray source for materials analysis systems, including but not limited to x-ray spectroscopy and x-ray fluoroscopy. It is another object of this invention to provide an increased efficiency x-ray source having significantly reduced power requirements, for use in materials analysis systems.
It is another object of this invention to provide a miniaturized x-ray source for materials analysis systems, including an electron source that can generate electrons with minimal heat loss. It is yet another object of this invention to provide a miniaturized x-ray source for materials analysis, in which an optical source is used to heat a thermionic cathode, instead of using conventional ohmic heating to heat a thermionic cathode. In this way, electrons can be produced in a quantity sufficient to form an electron current necessary for generating x-ray radiation at the target, while significantly reducing the requisite power requirements for the radiation source.